1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of transmitting channel information in a wireless communication system.
2. Related Art
Today, wireless communication systems have been evolving from the early voice-oriented services into high-speed, high-quality wireless data packet communication systems for providing data services and multimedia services. Moreover, for next generation wireless communication systems, standardization is in progress for high-speed, high quality wireless data packet services. High-speed downlink packet access (HSDPA) represents an evolution of the existing asynchronous international mobile telecommunications (IMT)-2000 standard and is an access scheme which is additionally provided for high-speed downlink data transmission. The HSDPA is classified into a frequency division duplex (FDD) HSDPA and a time division duplex (TDD) HSDPA. The TDD HSDPA can support a chip rate of 3.84 million chips per second (Mcps) or 1.28 Mcps. In the TDD HSDPA, uplink and/or downlink resources can be asymmetrically allocated. Thus, when downlink channels are allocated to the maximum extent possible, transmission can be achieved in a most efficient manner with the introduction of the HSDPA. The TDD HSDPA with the chip rate of 1.28 Mcps is generally referred to as time division-synchronous code division multiple access (TD-SCDMA).
The TDD HSDPA supports a hybrid automatic repeat request (HARQ) scheme and an adaptive modulation and coding (AMC) scheme. In the HARQ scheme, a receiver receives packet data and then informs a transmitter of whether the packet data is successfully received, so that the packet data can be retransmitted when necessary. In the AMC scheme, a modulation and coding scheme (MCS) level is dynamically changed according to a channel condition. When a user equipment (UE) has a good channel condition, a data rate can be increased by using a high modulation order and a high code rate. On the other hand, a relatively low modulation order and a relatively low code rate are used when the UE is located in a cell boundary. When using the AMC scheme, the UE has to feed back a channel quality indicator (CQI) to a Base Station (BS). The CQI is a representative value indicating downlink channel condition. The CQI may be used to directly report the MCS level. The CQI may be configured in various formats.
A high speed downlink shared channel (HS-DSCH) is a downlink transport channel for supporting the HSDPA. The HS-DSCH is associated with at least one shared control channel for HS-DSCH (HS-SCCH). The HS-SCCH is a downlink physical channel for carrying upper-layer control information for the HS-DSCH. A shared information channel for HS-DSCH (HS-SICH) is an uplink physical channel for carrying an acknowledgment (ACK)/negative-acknowledgment NACK) signal.
FIG. 1 shows a conventional structure of a burst for an HS-SICH. A burst transmitted on the HS-SICH has a duration of 84 bits in total, that is, 48 bits for a CQI and 36 bits for an ACK/NACK signal. In the TDD HSDPA, the CQI consists of a recommended modulation format (RMF) and a recommended transport block size (RTBS). The RMF is 1-bit information related to a modulation scheme. The 1-bit RMF is extended to 16 bits by using a repetition code. The RTBS is 6-bit information related to a coding scheme. The 6-bit RTBS is extended to 32 bits by using a Reed-Muller code. The ACK/NACK signal is 1-bit information. The 1-bit ACK/NACK signal is extended to 36 bits by using the repetition code.
In order for the AMC scheme to have improved efficiency, the CQI needs to be configured to indicate channel condition more accurately. This is because the channel condition changes due not only to white noise but also to various factors such as power variation of a received signal, a Doppler effect, and interference. The power variation is caused by fading. The Doppler effect is caused by shadowing, movement of a UE, and a frequently changing speed. The interference is caused by other users and multiple paths. In addition, in a channel environment where the UE has a high mobility, a downlink channel condition currently transmitted by the UE may differ from a downlink channel condition in which the BS transmits data in practice at a later time.
Accordingly, a channel path currently estimated by a receiver may differ from a channel path through which a transmitter transmits data at a later time. Therefore, in order to improve performance of the AMC scheme, there is a need to consider that a channel environment may change according to a moving speed of the UE.